Parallella

The Parallella computer is a high performance, credit card sized computer based on the Epiphany multi-core chips from Adapteva. Starting at $99, the Parallella can be used as a standalone computer, an embedded device, or as a component in a scaled out parallel server cluster.

VoCore2

A coin-sized Linux computer that is totally open-source, and costs $20. This is a small, low-cost, totally open-source (both the hardware and software) computer that's ideal for projects such as IoT or building your own custom router.